This invention concerns a device to compensate the elongation of at least two wire rods or round bars, the device being associated with a drawing assembly.
The device to compensate the elongation according to the invention is applied to all machines which process at the same time at least two wire rods or round bars, the machines being, for instance, bending/shaping machines or straightening machines.
The wire rods or round bars processed with these machines are produced by hot rolling or cold rolling or by drawing and consist of so-called round bars for building work of the type employed as reinforcements for concrete.
The state of the art covers bending/shaping machines, also called stirruping machines, which are fed with round iron bars so as to produce stirrups for building work.
These machines are generally fed with round iron bars stored in the form of coiled bundles, which are uncoiled and straightened before being bent.
These coiled bundles are generally formed at the end of the hot rolling cycle.
The stirruping machines generally include at their upstream end a straightening assembly consisting of a plurality of opposed and staggered rolls, in which the wire rod or round bar is stretched so as to remove the tensions and twists therein, and also include a drawing assembly consisting of at least one pair of opposed powered rolls the axes of which lie on a plane at a right angle to the plane of feed of the wire rod or round bar to be drawn.
The feed of the wire rod is provided by the pressure exerted by the pair of drawing rolls on the wire rod, this pressure having a considerable value so as to ensure enough drawing force to prevent any possible slipping of the wire rod on the rolls.
This pressure is such as to cause plastic deformation of the wire rod or round bar to the extent of creating an elongation substantially in proportion to the pressure.
A further elongation of the wire rod is caused also by the stretching arising from the straightening action applied to the wire rod.
This overall elongation of the wire rod may even reach a value of about 3 to 6%.
Where the machine is processing only one wire rod, this elongation does not create problems inasmuch as the means which measures the length is normally placed downstream of the aforesaid straightening and drawing assemblies, so that the length measured is the real length.
So as to increase the output, these stirruping machines have for some time now been generally enabled to process at least two wire rods or round bars at the same time.
In this case the straightening and drawing assemblies normally consist of a plurality of pairs of rolls containing multiple passes, or else of multiple rolls working on the same axis.
In this case the elongation resulting from the plastic deformations caused during the straightening and drawing leads to shortcomings which have so far not been overcome.
In fact, it has been found that the elongation caused by the pressure exerted by the straightening rolls, but above all by the drawing rolls, on the wire rods differs as between the two wire rods, thus involving different measurements and leading to the formation of stirrups having different dimensions which do not comply with the pre-set design specifications.
The difference of the elongation is caused by various factors which cannot be controlled in a precise and continuous manner by the machine operator and which combine together in a manner that cannot be foreseen, and are such as the following, in particular:
the difference of the diameters of the wire rods being processed, this difference being linked to the tolerances proper to the production process of the wire rod, causes very different elongations even though the difference between the diameters of the two wire rods being processed is much less than a millimetre; PA1 the difference of the tensions in the two wire rods, these tensions being removed or fixed during the straightening step, leads to different elongations; PA1 the different wear of the passes in the straightening rolls, but above all in the drawing rolls, causes a different deformation of the two wire rods and therefore a different elongation of the two wire rods; PA1 the mechanical plays both in the bearings of the rolls and in the guides for the sliding of the roll-holder slides lead to the application of different pressures on the two wire rods being processed.
To be more exact, where the rolls are fitted as cantilevers, the resilient yielding of the whole device has the effect that the drive shafts to which the rolls are fitted tend to spread apart, so that the wire rod closest to the support is squeezed to a greater extent and is therefore more elongated than the other wire rod.
All the above occurrences take place with all the pressure rolls applied, whether t hose rolls be straightening rolls or drawing rolls.
However, these deformations are greater with the drawing rolls since the pairs of drawing rolls consist of opposed rolls which are not staggered and at which the pressure exerted on the wire rods is greater and the effect is direct.
U.S. Pat. No. 3,392,896 discloses a floating ring cooperating with an inner drawing ring.
A system of resilient arms keeps the two pressure rolls pressed together.
This system is conceptually satisfactory for light pressures such as those which may be applied to coated electrical wires but is not suitable for iron bars which require, as we have said, considerable pressures for the drawing action.
DE-A-1.946.814 deals with a rolling mill stand having rolls supported as cantilevers, the stand having the purpose of processing one section at a time.
The problems of a rolling mill stand with rolls supported as cantilevers are conceptually different from those of a drawing assembly of the type of the present patent application.